text: For brevity in the equations below, we define the Perrin S factor.  For prolate spheroids (i.e., cigar-shaped spheroids with two short axes and one  long axis)
  where the parameter  is defined
  Similarly, for oblate spheroids (i.e., discus-shaped spheroids with two long axes and one short axis)
  For spheres, , as may be shown by taking the limit  for the prolate or oblate spheroids.